(1) Field of the Invention
The present invention relates to a secondary battery using a polymer active material. More particularly, the present invention relates to the composition and structure of an electrode using a polymer active material, as well as to a method for production of the electrode.
(2) Description of the Prior Art
Secondary batteries using a polymer material (e.g. a polyaniline or a polypyrrole) as an active material have been developed. In typical method for producing a electrode in such a secondary battery, a polymer active material, a conductivity-enhancing agent and an organic binder (e.g. a polytetrafluoroethylene) are kneaded using an appropriate solvent to obtain an electrode mixture; this electrode mixture is coated on a current collector (e.g. a metal foil), followed by drying, to form a film (or layer) of ordinarily about 20 to 100 xcexcm in thickness on the current collector. The resulting electrode is used as a positive electrode and/or a negative electrode; a separator is interposed between the positive electrode and the negative electrode, and a plurality of such combinations are laminated on each other to obtain a laminated type battery, or such a combination is wound to obtain a wound type battery.
In the above production method, however, during the drying of the coating film of electrode mixture formed on the current collector, the solvent contained in the coating film vaporizes, which incurs the contraction of the film and often generates cracks in the film. This phenomenon is more striking when the coating film is formed in a larger thickness; therefore, it is difficult to form a thick film. A large amount of the electrode active material is required in order to allow the produced battery to have a high energy density (i.e. a large energy per battery volume). In the above production method, the volume of the current collector in the battery is inevitably large; therefore, it is difficult to increase the amount of the active material to a level higher than now, within the restraint of limited battery volume. Thus, in the conventional production method, it has been difficult to further enhance the energy density of a battery using a polymer active material.
Conventional batteries have other problems. That is, they use a polyvinylidene fluoride, a polytetrafluoroethylene or the like as the binder. With such a binder, the surface of the polymer active material is covered with the binder and a thin film of the binder is formed on the surface of the current collector. Consequently, the battery has an increased electrical resistance and a decreased power density.
Conventional batteries still have other problems. That is, the electrode of battery has been limited to a sheet type, which limits the freedom of battery designing.
In JP-A-8-64200, it is described that by adding a plasticizer to a polymer active material, the adhesivity of the active material to an electrode substrate is increased, enabling a battery of higher energy density. In JP-A-8-64200, however, since the polymer active material, the plasticizer, etc. are coated in a liquid state to form an electrode member, it is difficult to form the film (or layer) in a large thickness, for the same reason as mentioned previously; therefore, the approach by JP-A-8-64200 is still insufficient with respect to the energy density of battery.
Thus, an electrode structure having a large ratio of electrode active material amount to current collector has been needed, but such an electrode has been unknown.
In view of the above-mentioned problem of the prior art, the object of the present invention is to provide a secondary battery using a polymer active material, which has a high energy density and a high power density and which permits significant freedom in battery designing. The another object of the present invention is to provide a method for producing a secondary battery of large energy density and large power density in a high freedom for battery designing, using a polymer active material.
Accordingly, the present invention is directed to a molded electrode comprising: (a) an electrode material comprising a polymer active material, a conductivity-enhancing agent and a plasticizer and (b) a current collector sheet; the electrode material and the current collector sheet molded into one piece, and the electrode material being in a thickness of 300 xcexcm to 9 mm and formed on at least one side of the current collector sheet.
Another aspect of the present invention is directed to a molded electrode comprising: (a) an electrode material comprising a polymer active material, a conductivity-enhancing agent and a plasticizer and (b) a plurality of current collector sheets; the electrode material and the current collector sheets formed into one piece, and the current collector sheets spaced each other in the thickness direction of the electrode.
Yet another aspect of the present invention is directed to a molded electrode comprising: (a) an electrode material comprising a polymer active material, a conductivity-enhancing agent and a plasticizer and (b) at least one current collector sheet; the electrode material and the current collector sheet formed into one piece, and the ratio of the volume of the electrode material and the volume of the current collector sheet being 30:1 to 100:1, provided the volume of the terminal portion of the current collector sheet is excluded from the volume of the current collector sheet. In this case, a plurality of the collectors and the electrode material may be molded into one piece.
Yet another aspect of the present invention is further directed to a method for producing a molded electrode, which comprises a step of: hot pressing (a) an electrode material which is a mixture of a polymer active material, a conductivity-enhancing agent and a plasticizer and (b) at least one current collector sheet.
Yet another aspect of the present invention is further directed to a method for producing a molded electrode, which comprises steps of: (i) hot pressing (a) an electrode material which is a mixture of a polymer active material, a conductivity-enhancing agent and a plasticizer and (b) at least one current collector sheet to form a molded material and (ii) conducting once or more times step of (ii-1) hot pressing the molded material, the same electrode material and a current collector sheet, or/and (ii-2) laminating and hot pressing a plurality of molded materials each produced as above, whereby forming a one-piece molded electrode comprising the electrode material and a plurality of current collector sheets spaced each other in the thickness direction of the electrode.